(a) Field of the Invention
This invention relates to the zone refining of cadmium and tellurium and, more particularly, to a method for the purification of cadmium and tellurium by zone refining in the presence of a gettering substance.
In the production of CdTe and CdHgTe compound semiconductors, it is necessary to use the elements with as high a purity as possible. Any iso-electronic impurities can generally be tolerated at relatively high levels, in the order of parts per million, but the carrier-generating impurities must be reduced to a level that preferably yields less than 10.sup.15 carriers/cm.sup.3 in the final semiconductor. For example, for efficient infrared detection, the carrier concentration in CdHgTe must be low, the mobility high and the excited-state lifetime must be long. The presence of impurities in the elements of CdHgTe has a negative effect on these characteristics.
(b) Description of the Prior Art
The purification of Cd, Te and Hg is described by Hirsch, H. E., et al. (Preparation of High Purity Cadmium, Mercury and Tellurium, Chapter 2 of Semiconductors and Semimetals, Vol. 18, Edited by Willardson, R. K. and Beer, A. C., Academic Press, 1981).
One of the methods for producing high purity, i.e. 69 or better, cadmium and tellurium is zone refining. Cadmium is one of the most difficult low melting metals to zone refine because of its low melting point, high thermal conductivity and high volume expansion. Because many impurities have segregation coefficients close to 1.0, only partial segregation is achieved. This is particularly so for Zn, Cu, K and Mg. Tellurium is better suited for zone refining because most impurities have very low distribution coefficients and can be easily removed. Those with distribution coefficients close to unity, namely Hg, Se, Cr, Na and Cd are not so efficiently removed. Cadmium and tellurium, therefore, are usually subjected to one or more than one zone refining operation to give single-, double-, triple-, or quadruple-zone-refined grades (SZR, DZR, TZR and QZR hereinafter). Cadmium and tellurium feed material is zone refined until the impurity equilibrium is established. The refined material is cropped, i.e. the section that meets 69 purity is separated from the leading and trailing ends and subjected to the same zone refining operation to give a DZR product with a reduced impurity content. Repeating the operation yields TZR material and refining this material gives QZR grade. The highly zone refined cadmium and tellurium, and high-purity mercury, when used in the production of CdHgTe, will consistently yield Cd.sub.x Hg.sub.1-x Te (x= 0.2) with carrier concentrations in the 10.sup.14 to 10.sup.15 range. Although such concentrations are adequate for the manufacture of most devices, further lowering of the carrier concentrations is desirable, which, however, requires the production of materials with still higher purities.
The purification of elements such as Sn, Si, Ge and Zn can be carried out by zone refining with an added solute or alloying agent for impurities or an added gettering substance. Pfann (Zone Melting, John Wiley & Sons, Inc., 1959) teaches that a substance containing an impurity can be purified by dissolving it in a solvent which enables a better separation of the impurity. The only example given by Pfann is the Sn-Si system, wherein a small quantity of Si back diffuses through an ingot of Sn and pure Sn is recovered.
According to U.S. Pat. No. 2,739,088, zone melting utilizes a two-solute system, wherein the molten zone is built up to the desired solute concentration. p-n and p-n-p junctions are formed by adding significant solute to the molten zone in solid, liquid or vapor form and in pure, alloyed or compound form. This method relates to doping and establishing solute concentration gradients.
According to U.S. Pat. No. 2 835 612, high melting point materials such as Si and Ge can be purified by zone refining with an alloying agent to alloy impurities, so that the substance is dissolved into the leading edge of the liquid region and is deposited from the trailing edge with different concentrations of impurities.
According to U.S. Pat. No. 3 047 380, Ge is purified by zone melting by adding an element which has an affinity for O.sub.2 greater than for Ge, forms an oxide and has a relatively small segregation constant, sweeping the element through the Ge and removing the oxide.
It is taught in AU Patent No. 273 393 that segregable impurities may be removed by zone refining from elements by applying a getter of low coefficient of solid-state diffusion at that end of the body to which segregable impurities are directed. Specifically, In is used as a getter for Cu in Ge purification. In is introduced at the tail end and the ultimate In distribution is attained quickly (negligible Cu back diffusion is assumed).
According to U.S. Pat. No. 4,165,249, a layer of relatively pure gettering material (Si) is applied to Ge and the Ge is zone refined with multiple passes.
A Russian paper (Chemical Abstracts, volume 84; 109005r, 1976) teaches the zone melting of Zn with an added active component such as Ge, Si, In, Sb, Bi, or Ca; a combination of Ge, Sn and Si giving the highest efficiency. Elements having partition coefficients &lt;0.5 are most suitable owing to strong interaction with impurities.
Thus, the prior art shows that zone refining of Cd and Te and that zone refining in the presence of a gettering substance for metals other than Cd and Te are well known.